9.1 Timing of activity of Setouchi volcanic belt and the rotation of the SW Japan arcK-Ar dating and paleomagnetic investigations were conducted on lavas from the Setouchi volcanic rocks in SW Japan, in order to understand the tectonics of characteristic high-Mg andesite (HMA) magmatism. New 53 K-Ar ages confirm that the Setouchi volcanism occurred at 15-12 Ma, especially the volcanism of intermediate to mafic rocks including HMAs took place at 13 Ma. A paleomagnetic direction of 13.9 Ma was determined for the volcanic rocks in NE Shikoku area. This data and previous ones with the ages in this study indicates that NE Shikoku area rotated CW about 19。 between 13.9 and 12.8 Ma after the area experienced CW rotation at about 15 Ma as a part of SW Japan. Such additional CW rotations are inferred from paleomagnetic data in the east Shikoku and Osaka areas. The additional rotations possibly imply that rotations of faulted blocks occurred in the main part of SW Japan, or that a part of SW Jap
… Morean including the eastern Shikoku and Osaka areas rotated CW between about 15 and 12.8 Ma, for which rotation angle was larger than that for the CW rotation of the hole SW Japan at about 15 Ma9.2 Thermal structure of the upper mantle and the melting of the subducting slabCharacteristic high-Mg andesite magmas were produced in the SW Japan arc at 13-15 Ma that was synchronous with the commencement of subduction of a very young (<11 m.y.) lithosphere of the Shikoku Basin. Numerical simulation <deleted> suggests that temperature at the surface of such a young subducting plate is high enough for partial melting both of the subducting sediments and oceanic crust. High-Mg andesite magmas were likely to be produced by interaction between silicic slab melts and the overlying mantle wedge. HMA magmas may be commonly produced in the Archean subduction zones under relatively high mantle temperature conditions, contributing to making continental crusts.9.3 The slab-melting and Setouchi magma genesisIn order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb-Nd-Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment component to the HMA magma generation. Mixing calculations using compositions of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H_2O-rich fluid nor an amphibolite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusual tectonic setting, including subduction of a newly-borne hence hot plate, may be responsible for melting of subducting sediments. Less